The Three-Quarter Cross Stitch is used in Counted Cross-Stitch designs to color half of a grid square on the diagonal. It consists of a Half-Cross Stitch and a Quarter-Cross Stitch.

When the grid square contains two colors, the second color is stitched with a Quarter-Cross Stitch. Notice how the graph doesn't tell you with which color to stitch the Half-Cross Stitch. It's usually up to your own judgement which color to emphasize.